Degradable plastics that decay in natural environment are drawing attention as an ideal way of disposing plastics. Among them, biodegradable plastics have heretofore been used disintegrating by the action of enzymes which the bacteria and true fungi release out of their bodies. Among the biodegradable plastics, aliphatic polyesters and, particularly, polylactic acids have been used, especially, in the field of packages and containers, since they are mass-produced on an industrial scale, are easily available and are environmentally friendly.
The polylactic acid (PLLA) is a resin using grain starches such as of corn as a starting material, and is a polymer using a product of starch fermented with lactic acid or L-lactic acid as a monomer, and is usually produced by a ring-opening polymerization or a direct polycondensation of a lactide which is a dimer thereof. The polymer is decomposed into water and a carbonic acid gas by microorganisms present in the natural world, and is drawing attention as a resin of the type of a completely recyclable system. The polymer, further, has a glass transition point (Tg) of about 60° C. which is close to the Tg of polyethylene terephthalate offering another advantage.
When it is attempted to apply a stretch mold article of polylactic acid to the use of, for example, producing containers for beverages, there still remain several problems that have to be solved. That is, to improve preservability of the content that is filled, it becomes necessary to conduct some form of sterilization by heating or pasteurization. However, the known stretch mold articles made from the polylactic acid lack heat resistance and are subject to be thermally deformed to a considerable degree even through heating at relatively low temperatures such as of washing with hot-water shower at the time of aseptic filling after having been sterilized with a drug.
To solve the above problems, the present inventors have proposed a method of draw-forming a hydroxyalkanoate resin comprising chiefly the polylactic acid followed by heat-setting in order to improve the oriented crystallinity of the formed article and to improve the heat resistance (patent document 1).
As the polylactic acid having improved heat resistance, there has also been known a stereocomplex polylactic acid comprising a poly-L-lactic acid of an L-lactic acid unit only and a poly-D-lactic acid of a D-lactic acid unit only (patent document 2).
Patent document 1: WO2003/008178
Patent document 2: JP-A-2006-36808